Palladium-Catalyzed Asymmetric Synthesis of Axially Chiral

Nov 19, 2010 - Stepwise application of the Pd-catalyzed SN2′ reaction and the desilylative SE2′ reaction to the ambivalent 2-bromo-1-silyl-1,3-die...
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ORGANIC LETTERS

Palladium-Catalyzed Asymmetric Synthesis of Axially Chiral Allenylsilanes and Their Application to SE2′ Chirality Transfer Reactions

2010 Vol. 12, No. 24 5736-5739

Masamichi Ogasawara,* Atsushi Okada, Velusamy Subbarayan, Sebastian So¨rgel, and Tamotsu Takahashi* Catalysis Research Center and Graduate School of Life Science, Hokkaido UniVersity, Kita-ku, Sapporo 001-0021, Japan [email protected]; [email protected] Received October 21, 2010

ABSTRACT

Stepwise application of the Pd-catalyzed SN2′ reaction and the desilylative SE2′ reaction to the ambivalent 2-bromo-1-silyl-1,3-dienes provides a novel route to the highly enantioselective construction of tertiary and quaternary propargylic stereogenic centers via axially chiral allenylsilanes.

Allenylsilanes are versatile intermediates for organic synthesis,1,2 and their axially chiral counterparts are capable of transferring the allenic axial chirality into newly formed propargylic stereogenic centers by a regio- and stereospecific SE2′ reaction with an appropriate electrophile.1,3 Although various protocols of preparing allenylsilanes have been reported,2 methods to prepare enantiomerically enriched axially chiral allenylsilanes have not been well developed.3 To the best of our knowledge, only three methods have been reported on the catalytic asymmetric synthesis of axially chiral allenyl(1) (a) Masse, C. E.; Panek, J. S. Chem. ReV. 1995, 95, 1293. (b) Fleming, I.; Barbero, A.; Walter, D. Chem. ReV. 1997, 97, 2063. (c) Marshall, J. A. Chem. ReV. 2000, 100, 3163. (d) Pornet, J. In Science of Synthesis; Fleming, I., Ed.; Georg Thieme Verlag: Stuttgart, 2002; Vol. 4, p 669. (e) Marshall, J. A. J. Org. Chem. 2007, 72, 8153. (2) (a) Westmijze, H.; Vermeer, P. Synthesis 1979, 390. (b) Danheiser, R. L.; Carini, D. J.; Basak, A. J. Am. Chem. Soc. 1981, 103, 1604. (c) Danheiser, R. L.; Carini, D. J.; Fink, D. M.; Basak, A. Tetrahedron 1983, 39, 935. (d) Fleming, I.; Terrett, N. K. J. Organomet. Chem. 1984, 264, 99. (e) Fleming, I.; Newton, T. W. J. Chem. Soc., Perkin Trans. 1 1984, 1805. (f) Danheiser, R. L.; Kwasigroch, C. A.; Tsai, Y.-M. J. Am. Chem. Soc. 1985, 107, 7233. (g) Danheiser, R. L.; Carini, D. J.; Kwasigroch, C. A. J. Org. Chem. 1986, 51, 3870. (h) Fleming, I.; Takaki, K.; Thomas, A. P. J. Chem. Soc., Perkin Trans. 1 1987, 2269. (i) Becker, D. A.; Danheiser, R. L. J. Am. Chem. Soc. 1989, 111, 389. (j) Kobayashi, S.; Nishio, K. J. Am. Chem. Soc. 1995, 117, 6392. (k) Weinreb, S. M.; Smith, D. T.; Jin, J. Synthesis 1998, 509. (l) Daidouji, K.; Fuchibe, K.; Akiyama, T. Org. Lett. 2005, 7, 1051. 10.1021/ol102554a  2010 American Chemical Society Published on Web 11/19/2010

silanes.4 This can be attributed to the lack of general routes to scalemic axially chiral allenes by asymmetric catalysis.4-7 Recently, we developed the Pd-catalyzed reaction of preparing various multisubstituted allenes,8a-h and the use of a (3) (a) Borzilleri, R. M.; Weinreb, S. M.; Parvez, M. J. Am. Chem. Soc. 1995, 117, 10905. (b) Suginome, M.; Matsumoto, A.; Ito, Y. J. Org. Chem. 1996, 61, 4884. (c) Marshall, J. A.; Adams, N. D. J. Org. Chem. 1997, 62, 8976. (d) Jin, J.; Weinreb, S. M. J. Am. Chem. Soc. 1997, 119, 5773. (e) Marshall, J. A.; Maxson, K. J. Org. Chem. 2000, 65, 630. (f) Guintchin, B. K.; Bienz, S. Organometallics 2004, 23, 4944. (g) Gonzalez, A. Z.; Soderquist, J. A. Org. Lett. 2007, 9, 1081. (h) Brawn, R. A.; Panek, J. S. Org. Lett. 2007, 9, 2689. (i) Reginato, G.; Mordini, A.; Tenti, A.; Valacchi, M.; Broguiere, J. Tetrahedron: Asymmetry 2008, 19, 2882. (j) Brawn, R. A.; Panek, J. S. Org. Lett. 2009, 11, 473. (k) Ohmiya, H.; Ito, H.; Sawamura, M. Org. Lett. 2009, 11, 5618. (l) Brawn, R. A.; Welzel, M.; Lowe, J. T.; Panek, J. S. Org. Lett. 2010, 12, 336. (4) (a) Tillack, A.; Michalik, D.; Koy, C.; Michalik, M. Tetrahedron Lett. 1999, 40, 6567. (b) Han, J. W.; Tokunaga, N.; Hayashi, T. J. Am. Chem. Soc. 2001, 123, 12915. (c) Nishimura, T.; Makino, H.; Nagaosa, M.; Hayashi, T. J. Am. Chem. Soc. 2010, 132, 12865. (5) For recent reviews: (a) Hoffmann-Ro¨der, A.; Krause, N. Angew. Chem., Int. Ed. 2002, 41, 2933. (b) Ohno, H.; Nagaoka, Y.; Tomioka, K. In Modern Allene Chemistry; Krause, N., Hashmi, A. S. K., Eds.; WileyVCH: Weinheim, 2004; p 141. (c) Ogasawara, M. Tetrahedron: Asymmetry 2009, 20, 259. (6) (a) de Graaf, W.; Boersma, J.; van Koten, G.; Elsevier, C. J. J. Organomet. Chem. 1989, 378, 115. (b) Matsumoto, Y.; Naito, M.; Uozumi, Y.; Hayashi, T. J. Chem. Soc., Chem. Commun. 1993, 1468. (c) Hayashi, T.; Tokunaga, N.; Inoue, K. Org. Lett. 2004, 6, 305. (d) Liu, H.; Leow, D.; Huang, K.-W.; Tan, C.-H. J. Am. Chem. Soc. 2009, 131, 7212.

proper chiral Pd catalyst furnished axially chiral allenes with high enantioselectivity.8i-m In this communication, we report the application of the Pd-catalyzed reaction to the asymmetric synthesis of axially chiral allenylsilanes. The newly developed substrates for the present study, 2-bromo-1-silyl-1,3dienes, possess both electrophilic and nucleophilic sites within single molecules. Stepwise application of the Pdcatalyzed SN2′ reaction and the desilylative SE2′ reaction to the ambivalent compounds provides a novel route to the asymmetric construction of chiral propargyl compounds in up to 94% ee (Scheme 1). Whereas the Pd-catalyzed reaction

Scheme 1

is tolerant of various functional groups, a variety of substituents could be introduced in the allenylsilanes. The 3,3dialkylallenylsilanes thus obtained can be converted to the corresponding propargylic compounds with an “all-carbon” quaternary stereogenic center by the SE2′ chirality transfer process. To realize the Pd-catalyzed synthesis of allenylsilanes, first, the preparation of yet unknown 2-bromo-1-silyl-1,3-dienes was examined. A series of bromosilyldienes 1a-f were obtained in good overall yields by the reaction sequence depicted in Scheme 2. Readily accessible β-silylenals/enones

Scheme 2

2 were converted into the corresponding R-bromo-β-silylenals/enones 3 by a successive Br2 and NEt3 treatment.9 Conversion of 3 into 1 was achieved by Peterson’s protocol: i.e., reaction of 3 with Me3SiCH2MgCl followed by an acidic treatment of the generated alcohols at 60 °C afforded 1 predominantly in (Z)-forms (>96%). The choice of the (7) For transition-metal-catalyzed kinetic resolutions of racemic allenes, see: (a) Noguchi, Y.; Takiyama, H.; Katsuki, T. Synlett 1998, 543. (b) Sweeney, Z. K.; Salsman, J. L.; Andersen, R. A.; Bergman, R. G. Angew. Chem., Int. Ed. 2000, 39, 2339. For transition-metal-catalyzed dynamic kinetic resolutions of racemic allenes, see: (c) Imada, Y.; Ueno, K.; Kutsuwa, K.; Murahashi, S. Chem. Lett. 2002, 140. (d) Trost, B. M.; Fandrick, D. R.; Dinh, D. C. J. Am. Chem. Soc. 2005, 127, 14186. (e) Imada, Y.; Nishida, M.; Kutsuwa, K.; Murahashi, S.; Naota, T. Org. Lett. 2005, 7, 5837. (f) Imada, Y.; Nishida, M.; Naota, T. Tetrahedron Lett. 2008, 49, 4915. (g) Nemoto, T.; Kanematsu, M.; Tamura, S.; Hamada, Y. AdV. Synth. Catal. 2009, 351, 1773. Org. Lett., Vol. 12, No. 24, 2010

methylenation method in the final step is important for the high yields of 1. While Peterson’s protocol was highly effective for the transformation giving 1 in up to 94% yield, the Wittig reaction of 3a with Ph3PdCH2 gave a complex mixture with less than 20% of 1a. The bromosilyldienes 1 are fairly stable and purified by silica gel chromatography and/or vacuum distillation. The bromosilyldienes 1 are excellent substrates for the Pdcatalyzed reaction with various carbon soft nucleophiles 4, and various allenylsilanes 5, including 1,3-disubstituted (Table 1, entries 1-9) and 1,3,3-trisubstituted (entries 10-12) allenylsilanes, were obtained in good to excellent yields in the presence of 2 mol % of a palladium catalyst generated in situ from [PdCl(π-allyl)]2 and dpbp.8a,10 The substrates 1 were consumed completely within 18 h, and the NMR and GC analyses of the crude reaction mixtures revealed concomitant formation of the dehydrobrominated enynes [Si]sCtCsCRdCH2 611 as byproducts. Similar dehydrobromination was not apparent in the analogous palladium-catalyzed reaction of 1-hydrocarbyl-2-bromo-1,3dienes.8 Apart from the formation of 6, the reaction was generally very clean and the allenylsilanes 1 were isolated in up to 93% yield by silica gel chromatography. Whereas the synthetic usefulness of 1 in the Pd-catalyzed reaction was proven as above, enantioselective synthesis of the axially chiral allenylsilanes was examined. Under the reaction conditions similar to those in our previous report,8i i.e., with a palladium catalyst (10 mol %) generated from Pd2(dba)4 and (R)binap, (R)-5am of 54% ee was obtained in 61% yield by a reaction of 1a with 4m at 23 °C (entry 13). The enantioselectivity was improved by the use of (R)-segphos8j,k,12 in place of (R)-binap, and (R)-5am was obtained in 86% ee (entry 14). Despite the higher enantioselectivity, the Pd/(R)-segphos species was prone to give 5 in lower yields. To gain reasonable yields of 5 with the Pd/(R)-segphos, higher temperatures (up to 80 °C) were required, but the loss of the enantioselectivity was minimal to negligible. A variety of axially chiral allenylsilanes of excellent enantiopurity (up to 94% ee in 87% yield in 5an; entry 15) were obtained under the optimized (8) (a) Ogasawara, M.; Ikeda, H.; Hayashi, T. Angew. Chem., Int. Ed. 2000, 39, 1042. (b) Ogasawara, M.; Ikeda, H.; Nagano, T.; Hayashi, T. Org. Lett. 2001, 3, 2615. (c) Ogasawara, M.; Ge, Y.; Uetake, K.; Fan, L.; Takahashi, T. J. Org. Chem. 2005, 70, 3871. (d) Ogasawara, M.; Fan, L.; Ge, Y.; Takahashi, T. Org. Lett. 2006, 8, 5409. (e) Ogasawara, M.; Okada, A.; Watanabe, S.; Fan, L.; Uetake, K.; Nakajima, K.; Takahashi, T. Organometallics 2007, 26, 5025. (f) Ogasawara, M.; Okada, A.; Nakajima, K.; Takahashi, T. Org. Lett. 2009, 11, 177. (g) Ogasawara, M.; Okada, A.; Murakami, H.; Watanabe, S.; Ge, Y.; Takahashi, T. Org. Lett. 2009, 11, 4240. (h) Ogasawara, M.; Murakami, H.; Furukawa, T.; Takahashi, T.; Shibata, N. Chem. Commun. 2009, 7366. (i) Ogasawara, M.; Ikeda, H.; Nagano, T.; Hayashi, T. J. Am. Chem. Soc. 2001, 123, 2089. (j) Ogasawara, M.; Ueyama, K.; Nagano, T.; Mizuhata, Y.; Hayashi, T. Org. Lett. 2003, 5, 217. (k) Ogasawara, M.; Nagano, T.; Hayashi, T. J. Org. Chem. 2005, 70, 5764. (l) Ogasawara, M.; Ngo, H. L.; Sakamoto, T.; Takahashi, T. Org. Lett. 2005, 7, 2881. (m) Ogasawara, M.; Fan, L.; Ge, Y.; Takahashi, T. Org. Lett. 2006, 8, 5409. (9) (a) Borrelly, S.; Paquette, L. A. J. Org. Chem. 1993, 58, 2714. (b) Fleming, I.; Marangon, E.; Roni, C.; Russell, M. G.; Chamudis, S. T. Can. J. Chem. 2004, 82, 325. (10) dpbp ) 2,2′-bis(diphenylphosphino)-1,1′-biphenyl. See: Ogasawara, M.; Yoshida, K.; Hayashi, T. Organometallics 2000, 19, 1567, and references cited therein. (11) Trost, B. M.; Tour, J. M. J. Org. Chem. 1989, 54, 484. (12) Saito, T.; Yokozawa, T.; Ishizaki, T.; Moroi, T.; Sayo, N.; Miura, T.; Kumobayashi, H. AdV. Synth. Catal. 2001, 343, 264. 5737

Table 1. Palladium-Catalyzed Synthesis of Axially Chiral Allenylsilanes 5a

entry

substrate 1

Nu-H 4

base

Pd-precursor (mol %)

P-P

temp (°C)

yield of 5 (%)b

% eec of 5 (config)d

1 2 3e 4 5 6 7 8 9 10 11 12 13 14 15 16e 17 18 19 20 21 22 23 24 25 26 27 28

1a 1a 1a 1a 1a 1a 1a 1b 1c 1d 1e 1f 1a 1a 1a 1a 1a 1a 1a 1a 1b 1b 1c 1d 1d 1d 1e 1f

4m 4n 4o 4p 4q 4r 4s 4m 4r 4m 4m 4r 4m 4m 4n 4o 4p 4q 4r 4s 4m 4n 4r 4m 4r 4r 4m 4r

NaH NaH NaH KH NaH NaH NaH NaH NaH NaH NaH CsOtBu NaH NaH CsOtBu CsOtBu CsOtBu CsOtBu CsOtBu CsOtBu NaH CsOtBu CsOtBu NaH CsOtBu CsOtBu NaH CsOtBu

[PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) [PdCl(π-allyl)]2 (1.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0) Pd2(dba)4 (5.0)

dpbp dpbp dpbp dpbp dpbp dpbp dpbp dpbp dpbp dpbp dpbp dpbp (R)-binap (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-segphos (R)-tBu-segphos

23 40 40 50 40 40 40 50 40 40 40 40 23 40 40 40 80 40 80 80 80 40 60 80 40 80 80 60

88 (5am) 73 (5an) 87 (5ao) 72 (5ap) 81 (5aq) 72 (5ar) 92 (5as) 93 (5bm) 76 (5cr) 81 (5dm) 88 (5em) 75 (5fr) 61 (5am) 62 (5am) 87 (5an) 82 (5ao) 64 (5ap) 67 (5aq) 63 (5ar) 63 (5as) 76 (5bm) 86 (5bn) 38 (5cr) 71 (5dm) 24 (5dr) 73 (5dr) 58 (5em) 73 (5fr)

s s s s s s s s s s s s 54 (R) 86 (R) 94 (R) 80 (R) 85 (R) 91 (R) 88 (R) 83 (R) 86 (R) 91 (R) 87 (R) 76 (R) 79 (R) 76 (R) 60 (R) 60 (R)

a The reaction was carried out in THF or dioxane (5 mL) for 18 h with 1 (0.50 mmol), 4 (0.65 mmol), and base (0.60 mmol) in the presence of Pd-precursor (5.0 µmol or 25 µmol) and bisphosphine (1.1 equiv to Pd). b Isolated yield by column chromatography. c Determined by chiral HPLC analysis (see Supporting Information for details). d The absolute configurations were deduced by the Lowe-Brewster rule (ref 12). e With 2.50 mmol of 4o (ref 8k).

conditions. The bulkiness of the silyl substituents in 1 showed virtually no influence on the enantioselectivity (entries 14-15, 19 vs 21-23). The Me3Si-substrate 1a generally gave the allenylsilanes in higher yields than 1b and 1c. The present Pd-catalyzed reaction is tolerant of additional functional groups, and a proelectrophile moiety (dimethyl acetal) could be installed in the allenylsilanes by the use of 4r and 4s (entries 19, 20, 23, 25, 26, and 28). The Pd-catalyzed reactions of the 3-alkyl-2-bromo-1-trimethylsilyldienes 1d-f were less enantioselective than those of 1a, and the corresponding 3,3-dialkylallenylsilanes were obtained in up to 79% ee (entries 24-28). Note that catalytic asymmetric synthesis of 3,3-disubstituted allenylsilanes is realized for the first time in this study. All the allenylsilanes obtained here are levorotatory, and their absolute configurations are deduced to be (R) by the Lowe-Brewster rule.13,14

The obtained (R)-5an of 94% ee was allowed to react with propanal dimethyl acetal at -78 °C in the presence of TiCl4 to give 73% of homopropargyl methyl ethers 7an, which consists of (S,S)- and (S,R)-diastereomers in a ratio of 4:1 (Scheme 3,

Scheme 3

(13) (a) Lowe, G. Chem. Commun. 1965, 411. (b) Brewster, J. H. Top. Stereochem. 1967, 2, 1. 5738

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top).15-17 The absolute configurations of the propargyl carbons in 7an were deduced to be (S) based on the known anti stereochemistry in the SE2′ reactions of allenylsilanes.15,16 Their enantiopurities, determined by chiral HPLC analysis, were found to be 94% ee for both diastereomers, demonstrating that the axial chirality of the allenylsilane was completely transferred to the propargylic stereogenic centers in 7an in the SE2′ reaction with the acetal. In the same way, a treatment of (R)-5an (94% ee) with Selectfluor, an electrophilic fluorinating reagent, in acetonitrile at 23 °C afforded the levorotatory chiral propargylic fluoride 8an in 94% ee in 77% yield with retention of the original enantiomeric purity in 5an (Scheme 3, bottom). The absolute configuration of (-)-8an was deduced to be (S) based on the stereospecific anti SE2′ mechanism of the desilylative fluorination.17 Protodesilylation of the 3,3-dialkylallenylsilanes took place via the anti SE2′ pathway and resulted in the formation of a propargylic tertiary stereogenic center in the products with axial-to-central chirality transfer (Scheme 4).18 The reactions

Scheme 4

of the optically active allenylsilanes (R)-5dm (76% ee) or (R)-5em (60% ee) with BF3·CH3CO2H in dichloromethane gave the corresponding propargyl compounds (R)-9dm (74% ee, 61% yield) or (R)-9em (59% ee, 60% yield), respectively. Although a slight decrease in enantiopurity was detected in the protodesilylation, the axial-to-central transfer of chirality was still greater than 97%. The chirality transfer in protodesilylation has been unique and could be realized by the use of the 3,3-disubstituted allenylsilanes. Treatment of the acetal-tethered allenylsilane (R)-5ar of 88% ee with TiCl4 promoted cyclization via the SE2′ pathway. The five-membered carbocycles were obtained in 71% yield as a mixture of two diastereomers 10ar and 11ar in a 1:2 molar ratio. Both diastereomers retained the enantiopurity of (R)-5ar within experimental error (Scheme 5). The highly enantioenriched six-membered carbocycles (14) Previously, the validity of this deduction was confirmed by the asymmetric total synthesis of a naturally occurring allene of known configuration utilizing the Pd-catalyzed reaction as a key step (ref 8k). (15) (a) Buckle, M. J. C.; Fleming, I. Tetrahedron Lett. 1993, 34, 2383. (b) Buckle, M. J. C.; Fleming, I.; Gil, S.; Pang, K. L. C. Org. Biomol. Chem. 2004, 2, 749. (16) The relative configurations of the homopropargyl carbons in 7an were tentatively assigned as in Scheme 3 by comparison of their 1H NMR data with those of analogous compounds of which configurations are known; see: Favre, E.; Gaudemar, M. J. Organomet. Chem. 1975, 92, 17. (17) (a) Carroll, L.; Pacheco, M. C.; Garcia, L.; Gouverneur, V. Chem. Commun. 2006, 4113. (b) Carroll, L.; McCullough, S.; Rees, T.; Claridge, T. D. W.; Gouverneur, V. Org. Biomol. Chem. 2008, 6, 1731. (18) Fleming, I.; Terrett, N. K. Tetrahedron Lett. 1983, 24, 4153.

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Scheme 5

10as (82% ee) and 11as (83% ee) were prepared in 86% yield in a 1:6 molar ratio from (R)-5as (83% ee) in the same way with the nearly complete transfer of chirality. The absolute configurations of the propargyl carbons in 10 and 11 were assigned to be (S) as in the case of 7an.13 The relative configurations of the adjacent stereogenic centers were determined by 1H NMR NOE experiments, and thus the major enantiomers in the cyclized products were (S,S)10 and (S,R)-11 as depicted in Scheme 5. The reaction sequence could be applied to the asymmetric construction of quaternary chiral centers bonded to four carbon substituents.19 The axial chirality of the trisubstituted allenylsilanes (R)-5dr (76% ee) and (R)-5fr (60% ee), which were prepared by the Pd-catalyzed asymmetric reaction of 1d and 1f, was transferred to the quaternary carbons in 10dr, 11dr, 10fr, and 11fr by the TiCl4-promoted intramolecular SE2′ reaction with complete retention of the enantiomeric purity in (R)-5dr and (R)-5fr (Scheme 5). These results demonstrated that, with a proper R substituent in 1, the asymmetric reaction shown in Table 1 could be an alternative to catalytic asymmetric synthesis of all-carbon quaternary centers. In summary, we have developed a novel route to axially chiral allenylsilanes by asymmetric catalysis. The axial chirality in the allenylsilanes, which were prepared with high enantioselectivity (up to 94% ee), was transferred to tertiary and quaternary stereogenic centers without loss of their enantiomeric purity. Acknowledgment. A part of this work was supported by a Grant-in-Aid for Scientific Research on Priority Areas “Advanced Molecular Transformations of Carbon Resources” from MEXT, Japan, and NOASTEC (to M.O.). Supporting Information Available: Detailed experimental procedures and compound characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. OL102554A (19) (a) Corey, E. J.; Guzman-Perez, A. Angew. Chem., Int. Ed. 1998, 37, 388. (b) Douglas, C. J.; Overman, L. E. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5363. (c) Trost, B. M.; Jiang, C. Synthesis 2006, 369. (d) Quaternary Stereocenters; Christoffers, J., Baro, A., Eds.; Wiley-VCH: Weinheim, 2004.

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